Computer chip evolution continues to be driven by a need to develop higher speed, increased functionality, and improved reliability. This need requires the development of new chip architectures with higher integration density, i.e., more devices per chip, and miniaturized design rules having smaller device features.
With the introduction of ultra-large scale integration (ULSI), where device features are in the sub-quarter-micron dimension, new interconnect metals and associated processes are being heavily investigated for potential incorporation in the fabrication of the ULSI multilevel metallization (MLM) schemes required. Of these interconnect metals and processes, the chemical vapor deposition (CVD) of copper appears to be one of the most promising approaches. Copper exhibits a lower resistivity (bulk resistivity of 1.68 .mu..OMEGA.cm) than aluminum, which would be expected to yield significant reduction in RC time delay, where RC=resistance.times.capacitance. It is also predicted to display enhanced electromigration and stress resistance which would lead to higher reliability and improved device performance. In addition, CVD displays an intrinsic potential for void-free filling of aggressive via and hole structures at near bulk resistivity and high growth rates.
CVD growth of copper has been reported using both the copper (I) and copper (II) classes of source precursors, (I) and (II) referring to the respective oxidation states of copper, i.e., Cu.sup.+1 and Cu.sup.+2. Each class of precursors has its marked characteristics and specific decomposition pathways. Preferred copper (I) precursors are liquid at room temperature, which ensures their controlled and accurate delivery to the CVD reactor. Additionally, copper (I) precursors exhibit high volatility, leading to copper deposition at high growth rates. It is also accepted that these sources decompose by disproportionation which is illustrated, for example, in the reaction formula shown below where the reaction pathway proceeds from left to right: EQU 2Cu.sup.I (HFAC)L.revreaction.2Cu.sup.I (HFAC)+2L.revreaction.Cu.sup.o +Cu.sup.II (HFAC).sub.2 (I)
In the above formula (I), L is a Lewis base neutral ligand such as trimethylvinylsilane (TMVS) and HFAC is hexafluoroacetylacetonate. The ligand base L is weakly coordinated to the copper(I)(hexafluoroacetylacetonate) molecule, and easily uncouples from the remainder of the molecule, leaving copper(I)(hexafluoroacetylacetonate) as an intermediate species. The rate-limiting step in the Formula (I) is thus the adsorption of copper(I)(hexafluoroacetylacetonate) to substrate surface sites, where electron exchange between metallic centers is easily catalyzed by electrically active sites of the conducting substrate surface leading to the second reaction step in Formula (I) above. The disproportionation reaction is catalyzed by metallic or other electrically conducting portions of the substrate surface at temperatures from 110.degree. C. to 190.degree. C. This leads, in principle, to selective copper deposition on the conducting, but not insulating, areas of the substrate. See, e.g., U.S. Pat. No. 5,098,516 of Norman et al.
Molecular recombination, as shown in Formula (I) moving in the reverse direction from right to left, is also feasible in the case of copper(I) source precursors. The occurrence of molecular recombination has been demonstrated through a variety of studies, including the successful etching of deposited copper using a mixture of copper(II)(hexafluoroacetylacetonate).sub.2 and trimethylvinylsilane at 140.degree. C., and the demonstration of reversibility of copper growth from copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane). See, e.g., M. Naik et al., Thin Solid Films 262, p. 62 (1995) and Norman et al., J. Physics, IV, p. C2-271 (1991).
It is also believed that, if hydrogen is present during the reaction, the decomposition of the copper(I) source precursors could additionally occur through a reduction pathway as shown in formula (II): ##EQU1##
Copper(II) sources, in contrast, are easily reduced by hydrogen either thermally (at temperatures above about 250.degree. C.) on a catalyzing substrate or in a plasma environment (at temperatures as low as 100.degree. C.), to yield pure copper according to a process as shown in Formulae (III) and (IV) below, for the case of anhydrous copper(II)(hexafluoroacetylacetonate).sub.2 : EQU H.sub.2 .fwdarw.2H (III) EQU Cu.sup.II (HFAC).sub.2 +2H.fwdarw.Cu.sup.o +2 H(HFAC) (IV)
As a copper source, copper(II)hexafluoroacetylacetonate).sub.2 has demonstrated the ability to produce high electrical quality copper films, with as-deposited resistivities of 1.7 .mu..OMEGA.cm. Copper (II) sources are also known to be robust, non-air-sensitive, and easy to synthesize and handle.
Unfortunately, each class of precursors suffers from some shortcomings which inhibit its adaptability to semiconductor manufacturing practices. Copper (I) sources are reported to be highly air-sensitive and exhibit poor transport properties, with pure copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane), for example, exhibiting a narrow delivery window of about 55.+-.5.degree. C. Lower temperatures cause precursor condensation in the delivery lines, while higher temperatures lead to its premature decomposition during transport to the reactor. Additionally, the synthesis of copper (I) sources is not straightforward, and tends to produce films of lower electrical quality. For example, thermal CVD from pure copper(I)(hexafluoroacetylacetonate) (trimethylvinylsilane) has been unsuccessful at producing as-deposited copper resistivities lower than 2.0 .mu..OMEGA.cm.
Efforts to resolve these issues have focused on doping pure copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane) with chemical additives to improve its stability and performance. In this context, the addition of trimethylvinylsilane to the source precursor led to more stable delivery to the CVD reactor at appreciable flow rates, while the incorporation of H(HFAC) produced smoother copper films at higher growth rates than in the case of copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane) without additives See, e.g., Norman et al., Thin Solid Films, 262, p. 46 (1995). Also, the inclusion of water vapor in the CVD process produced significant enhancement in copper growth rates. See, e.g., Gelatos et al., Appl. Phys. Lett. 63, p. 2842 (1993) and Norman et al., Process Semiconductor Research Corporation Workshop On Copper Interconnect Technology, Troy, N.Y. (August 1993), SRC, Research Triangle Park, North Carolina (1993), p. 130. However, these approaches led to increased precursor cost and complexity, and have not yet produced the precursor reliability and suitability for manufacture desired.
From a processing perspective, the disproportionation reaction involved in the thermal CVD of copper from copper (I) sources is inefficient, with one copper atom being deposited on the substrate for every two copper atoms provided to the reaction, as can be seen in Formula (I) for copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane). The second copper atom is removed from the reaction zone in the form of a copper(II).beta.-diketonate precursor, such as copper(II)(hexafluoroacetylacetonate).sub.2.
Copper (II) sources, in contrast, have lower volatility than copper (I) sources, which leads to reduced copper growth rates and the inability to completely fill the aggressive topographies associated with emerging semiconductor integrated circuit technologies. Also, these sources tend to be solid at room temperature, which poses significant challenges to their accurate and controllable delivery to the reaction zone. Their decomposition in thermal CVD processing tends to occur at higher temperatures than their copper (I) counterparts, with the decomposition of copper(II)(hexafluoroacetylacetonate).sub.2, for example, occurring above about 250.degree. C. Conventional plasma-assisted CVD in a hydrogen plasma successfully reduced these temperatures to below 150.degree. C. However, plasma-promoted CVD without a bias substrate failed to achieve complete fill of aggressive device topographies. The introduction of a bias substrate is expected to enhance the probability of precursor species adsorption and re-emission within such topographies, leading to improved fill characteristics. However, it adds a level of process and equipment complexity which is technologically and economically viable only if no simpler solutions are found. As such, there is a need in the art to develop a simpler, more economical process. Plasma-assisted CVD is described in A. E. Kaloyeros et al., "Copper CVD for Multilevel Interconnect in ULSI," MRS Bulletin. vol. 18 (1993), p. 22; B. Zheng et al., "New Liquid Delivery System for CVD of Copper from .beta.-diketonate Precursors," Appl. Phys. Lett., vol. 61 (1992), p. 2175 and H. Li et al., "remote Plasma CVD of Copper for Applications in Microelectronics," J. Vac. Sci. Technol., vol. B10 (1992), p. 1337.
There is a need in the art to develop a copper source chemistry and associated CVD deposition process which provides planarized and void-free copper fill of aggressive sub-quarter-micron structures with as-deposited resistivity near the bulk resistivity, particularly structures of .ltoreq.0.25 .mu.m with an aspect ratio of 4:1 and beyond, where the aspect ratio is defined as the height of a hole or via measured longitudinally divided by the width or diameter of the hole or via measured transversely. Such criteria must be met in order to provide industrially acceptable wafer throughput and associated reliability, including the ability to process up to 5,000 wafers without tool downtime resulting from precursor condensation or premature decomposition in the delivery lines, or tool contamination from precursor species and associated by-products.
There is further a need in the art for a copper source chemistry which possesses high volatility, significant vapor pressure, and a high vaporization and sublimation rate to ensure an industrially acceptable wafer throughput. It would be particularly of interest to develop such a copper source in liquid form given the advantages of being able to use liquid delivery systems, such as the MKS direct liquid injection (DLI) system, which would ensure highly accurate, controllable and reproducible precursor liquid delivery into the reaction zone. There is also a need for a chemically and environmentally stable copper source which allows for extended shelf life and ease in transport and handling. Such a copper source chemistry should also be economical to use with respect to synthesis, mass production, disposal and the cost of start-up materials.
Multiple distillation steps are required to produce pure copper(I)(hexafluoroacetylacetonate)(trimethylvinylsilane) for use in forming films, and additional distillation steps significantly increases the cost of the precursor. Such cost problems become even more significant when doping using chemical additives is undertaken to improve the stability and performance of the pure copper(I)(hexafluoroacetylacetonate) (trimethylvinylsilane). As such, there is also a need in the art for a copper source chemistry which can be utilized as synthesized without the time and yield loss associated with distillation and doping steps thereby reducing the overall process cost.
Prior art selective copper CVD processes are not feasible within the constraints imposed by current manufacturing practices in view of the need to use ultra-clean sample preparation and associated processing conditions to ensure selectivity. Therefore, as an alternative, there is a need in the art for a CVD process and copper source precursor which provides a copper growth process independent of the type of substrate used. It would also be desirable for such a process to be able to deposit copper on various substrates, including non-conducting surfaces, such as amorphous diffusion barriers, or adhesion promoters which tend to be electrically semiconducting or insulating. As noted above, the disproportionation reaction which occurs in thermal CVD of copper from copper(I) sources "wastes" one copper atom in the form of a Cu.sup.II .beta.-diketonate precursor for every two copper atoms provided to the reaction. As such, there is further a need in the art for a copper source precursor which incorporates both copper atoms from the disproportionation reaction in the growing copper film.